Heater bodies and their production



May 15, 1956 w, GLASER 2,745,928

HEATER BODIES AND THEIR PRODUCTION Filed June 5, 1953 v2 [Af j/ (5% 705%)63: a

IN VEN TOR. E 41/. 6'4 45x5 HEATER BODIES AND THEIR PRODUCTION Frank W.Glaser, New York, N. Y., assignor to American Electro Metal Corporation,Yonkers, N. Y., a corporation of Maryland Application June 3, 1953,Serial No. 359,343 Claims priority, application Austria October 6, 19523 Claims. (Cl. 201 613) This invention relates to electric resistorheater bodies, and more particularly to such bodies which are formedwith molybdenum disilicide and to the production of such heater bodies.

It has long been known that molybdenum disilicide MoSiz has unusuallydesirable properties which would make it an ideal material for electricresistor heater bodies.

However, in the past, electrical heater rod bodies for electric furnacesand the like have in the past been made substantially out of siliconcarbide, such heater bodies being known commercially as Globar bodies.The known silicon carbide heater bodies have a maximum operatingtemperature of about 1400 C., and they are made with a resistivity of2000 to 5000 microhm centimeters because with lower resistivity it isdiflicult to supply such heater bodies with the much larger currentsthat would be required to bring them to the desired high operatingtemperatures.

Because of its relatively low electrical resistivity of about 22 microhmcentimeters at room temperatures, no practical way was found in the pastfor utilizing molybdenum disilicide as electrical heater bodies.Cemented bodies of molybdenum disilicide are very brittle and fragileand proposals to increase the resistivity of molybdenum disilicide byadditions of molybdenum aluminides, and/or high melting oxides, such aszirconium oxide, thorium oxide, which become conducting at hightemperatures, and/ or other high melting oxides, such as aluminum oxide,beryllium oxide, silicon oxide, likewise failed because these additionsresult in a further increase in the objectionable brittleness ofmolybdenum disilicide.

One phase of the present invention is based on the discovery that bycombining molybdenum disilicide with chromium boride CrBz, there isobtained a material of considerably smaller resistivity than molybdenumdisilicide, the new material being also of a lower order of brittlenessthan molybdenum disilicide.

The foregoing and other objects of the invention will be best understoodfrom the following description of exemplifications thereof, referencebeing made to the accompanying drawings, wherein the single figure is anelevational view with parts in section showing one form of heater bodyexemplifying the invention.

Bodies of pure molybdenum disilicide MoSiz which melts at about 1880 C.can be readily molded from MoSiz powders by powder metallurgy andceramic techniques. Because of their high stability and resistance tocorrosion in oxidizing atmospheres at 1700 to l800 C., many proposalshave been made in the past for using cemented MOSiz material in electricresistance heaters for operation at heating temperatures of about 1700C. However, because MOSiz has a relatively low electrical resistivity ofonly 22 microhm centimeters at room temperature, heater bodies of thismaterial would have to be made with a very thin cross-section, such asthin tubing or ribbon.

In addition, such cemented MoSiz material is very brittle andaccordingly, thin heater bodies of such material are i nited StatesPatent O also impractical because of their excessive criticalbrittleness.

One phase of the invention electric heater bodies of similar physicalcharacteristics as MoSiz, but about 10 times higher resistivity will beobtained by combining molybdenum disilicide with chromium boride CrBz ina critical range of proportions and that such combined new material isof a lower order of brittleness than any other combination of molybdenumdisilicide with other additional ingredients that might reduce theelectrical conductivity thereof, including additions of the oxides,nitrides, silicides or other borides of the transition metals of thefourth, fifth and sixth groups of the periodic table.

An investigation has shown that MoSiz and chromium boride are mutuallysoluble over the range containing 5% to about 50% CrBz and that bycombining MOSiz with 5% to 50% CrBz, there is obtained a heaterresistance material of a lower order of resistivity without increasingthe brittleness of the material compared to MoSiz. (Throughout thespecification and claims, all proportions are given by weight, unlessotherwise stated.) Superior results are obtained by combining 5% to 50%CrBz with MoSiz. Within this critical range of proportions the stabilityof either of the components MoSiz and CrBz is not affected by any sidereaction and the combined material may be readily produced in a wellcontrolled manner.

As distinguished therefrom, if molybdenum disilicide is combined withother metal borides or with the oxides, nitrides and/ or silicides ofthe metals of the fourth, fifth and sixth groups of the periodic table,the resulting materials are of such high brittleness as to render themutterly impractical for use in electrical heater bodies.

Investigations have shown that electrical resistivity of MOSiz at roomtemperature, increases from 22 microhm centimeters to about 200 microhmcentimeters, or by a factor of 10, as the content of CrBz is increasedfrom 0 to 5 and that throughout this range, the MoSiz and CrBz form aseries of solid solutions which exhibit metallic characteristics.

If the content of CrBz is increased above 50%, the resistivity of theresulting material decreases. On the other hand, if the molybdenumdisilicide is combined with other elements for decreasing itsresistivity, side reaction takes place which lead to the decompositionof the molybdenum disilicide compound. By way of example, if aluminumoxide A1203 is added to MOSiz and heated with it to elevatedtemperatures, side reaction takes place leading to the decomposition ofthe molybdenum disilicide into MoAlSi-O type compounds wherein the Si-Ophase is the most stable. The strength of such other body aggregatedecreases from the original strength of pure molybdenum disilicide.Thus, whereas, pure MoSiz has a transverse rupture strength of 60,000 p.s. i. (pounds per square inch) when it is combined with aluminum oxide,its transverse rupture strength decreases to about 13,000 to 15,000 p.s. i. Furthermore, the equilibrium conditions for such aggregate cannever be achieved, and the value of the transverse rupture strength isin a continuous flux.

An outstanding advantage of bodies of the invention which combine MoSizwith 5% up to 50% CrBz is the fact that such combined solid solutionbody exhibits essentially the structure of the CIBz compound. Thisfactor is of great advantage since at elevated temperature, thetemperature coefficient of resistivity of the MoSiz increases linearlywith temperature. On the other hand, MoSi2, if alloyed or combined inaccordance with the invention with CrBz over the range of theproportions up to 50% CrBz, gives a body with a temperature coefficientof resistance which approaches 0 at about 1200 C. and does not changewith a further increase of the temperature up to about 1700 to 1900 C.,the new material melting only at about is the discovery that superior w2000" C.v The negligible change in the resistivity of the combinedmaterial of the invention over the temperature range from about 1200 to1900 C. and higher, is of great practical importance for electric heaterbodies formed of such material.

By way of example, there will now be described one satisfactory.procedure for. producing. such body of' the invention. A powder mixturecontaining 10% CrBzwith 90% MOSis is hot pressed. at 1500 C. into acylindrical compact. The compact is then heated. in hydrogen for 15hours at 1300 C. and thereafter comminutcd, yielding powder particleswhich contain the desired proportions of CrBz and MoSiz in completelyalloyed solid solution form.

The alloy powder particles so obtained. are then ready for formingheater bodies. To this end, the alloy powder particles may be coldpressed into the desired shape, presintered, then further shaped to giveit final shape, followed by sintering into the desired shaped heaterbody, such as a heater rod. The alloy powder may be also shaped into thedesired heater body by extruding it after first mixing it with anextrusion. vehicle such as methyl cellulose, followed by a sintering invacuum. When making the desired heater body by the extrusion process,the extruded rods should be sintered while they are suspended in ahanging position to avoid distortion.

In producing out of t .e alloy powder the desired heater bodies byextrusion, it is important that the. heating cycle of the sintcringoperation should be very rapid. In other words, the final sinteringtemperatures should be reached from room temperature within about 10minutes. If the heating cycle is slow, the organic. binder which ismixed with the alloy powder for carrying on the extrusion, will departrapidly, leaving a relatively weak porous structure, thus endangeringthe production of satisfactory heater rod bodies. such as methylcellulose, should be de-aired before mixing it with the alloy powder, asthe entrapped air interferes with the rapid heating of the rods and alsocauses blowups. In lieu of the methyl cellulose, other binders may beused such as ethyl cellulose, cellulose acetate with appropriatesolvents.

The single figure indicates schematically the body exemplifying theinvention. An elongated cylinder of MoSi-z combined with 40% CrBz formsthe heater body 11 which is provided at its ends with water-cooledelectrode terminals 12.

Inasmuch as the addition of to 50% of CrBz to MoSiz increases theresistivity of the resulting combined material only by a factor of 10,it is desirable to form heater bodies made of such material of only thincross-section so that no excessively large currents be required forbringing the heater body material to the operating temperature of about1700 C. According to the invention, desired heater It is also importantthat the extrusion vehicle,

bodies having such thin heater body layer of MOSiz con taining 10 toCrBz are made as follows: A relatively rigid supporting body, such as arod of ceramic material is covered with a surface layer of allowedpowder particles of MoSiz containing in solid solution 5 to 50% CrBz.The material for the applied coating layer may be prepared by mixing thealloy powder with a suspension medium such as methyl cellulose and thepowder mixture so prepared is then applied as a uniform coating layer tothe ceramic rod. The coated ceramic rod is then sintered at elevatedtemperature to solidify the applied coating layer and drive off themethyl cellulose in the manner analogous to that described above ispreparing heater rods by extrusion.

Heaters of the invention formed in the manner described above, aresupen'or to conventional silicon carbide heaters because they may beoperated for prolonged periods of time without corrosion at temperaturesmuch higher than the maximum temperature of 1400 at which such siliconcarbide heaters may be used, the heaters of the invention cost only afraction of the cost of such silicon carbide heaters.

The features and principles underlying the invention described above inconnection with specific exemplifiestions, will suggest to those skilledin the art many other modifications thereof. It is accordingly desiredthat the appended claims be construed broadly and that they shall not belimited to the specific details shown and described in connection Withexemplifications thereof.

I claim:

1. In an electric heater device, a heater resistor body having spacedmetallic electrodes for passing heating current therethrough, said bodyconsisting essentially of molybdenum disilicide MoSiz containing 5% to50% CrBz in solid solution.

2. A heater device as claimed in claim 1, the molybdenum disilicideMoSiz of said body containing 10% to 50% CrBz and having a negligiblylow thermal coefficient of resistivity at temperatures between about1300 to 1700 C.

3. A heater device as claimed in claim 1, the molybdenum disilicideMoSiz of said body containing 40% to 50% CrBz and having a negligiblylow thermal coefiicient of resistivity at temperatures between about1300 to 1700 C.

References Cited in the file of this patent UNITED STATES PATENTS2,412,373 Wejnarth Dec. 10, 1946 2,622,304 Cofit'er Dec. 23, 19522,650,903 Garrison et al. Sept. 1, 1953 2,665,474 Beidler Jan. 12, i954

1. IN AN ELECTRIC HEATER DEVICE, A HEATER RESISTOR BODY HAVING SPACEDMETALLIC ELECTRODES FOR PASSING HEATING CURRENT THERETHROUGH, SAID BODYCONSISTING ESSENTIALLY OF MOLYBDENUM DISILICIDE MOSI2 CONTAINING 5% TO50% CRB2 IN SOLID SOLUTION.